1. Field of the Invention
The present invention relates to a starter for starting an engine.
2. Description of the Prior Art
Starters (coaxial-type starter) have been known which have the structure in which an electromagnetic switch, an overrunning clutch with a pinion engaged with a ring gear and a plunger (moving core) are set coaxially with respect to an output shaft.
This type of starter works as follows.
Specifically, when current flows through an exciting coil of the electromagnetic switch, the plunger is attracted by an exciting core of the electromagnetic switch and a moving contact is brought into contact with a fixed contact a short while after the attraction and shift of the plunger, to supply power to a d.c. motor thereby rotating the output shaft via a shaft (a motor shaft) and reduction mechanism and the like. By this rotation, the overrunning clutch which is spline-joined with the output shaft shifts toward the ring gear whereby the pinion is engaged with the ring gear to start an engine.
The above plunger has a cylinder form, is arranged so as to enclose the outside periphery of the output shaft and shifts the overrunning clutch toward the ring gear.
The overrunning clutch is provided with a thrust spline having a cylinder section with an inside periphery on which a helical spline section which is spline-joined with a helical spline section formed on the output shaft and also constitutes a so-called one-way clutch.
In such a clutch as aforementioned, the output shaft and the thrust spline of the overrunning clutch which is spline-joined with the output shaft are usually made from hardened steel.
However, the use of the output shaft and thrust spline formed from hardened steel causes a lot of magnetic flux to leak from the plunger to the output shaft and from the output shaft to the thrust spline and hence insufficient attractive force (initial attractive force) is obtained at the start when the resting plunger is attracted, specifically, posing the problem of insufficient force for attracting the plunger.
In the above case, it is possible to reduce the leakage of the magnetic flux by designing a large interval between the output shaft and the plunger (an air gap between the inside periphery of the plunger cylinder and the outside periphery of the output shaft). This however poses the problem that the size of the starter in the radial direction is increased with the result that the starter is large in size.